Geology and Hydrothermal Alteration of Recently Drilled Four Wells At Aluto Langano Prospect

The Aluto Langano Geothermal Field is one of the various geothermal prospects in Ethiopia. The prospect is located in the central part of the Main Ethiopian Rift system. It has been the subject of various geothermal investigations. Early investigations in the prospect included: surface explorations, the drilling of eight deep exploratory wells in the 1980’s and the drilling of additional two appraisal deep wells in the 2010’s. The initial exploratory drillings have discovered sufficient steam in four productive wells which have been running a pilot plant of 7.3 MW. The reservoir is liquid dominated with maximum temperature of 335ᵒ C. Current project at Aluto includes: the drilling of 10 production wells for potential steam discovery for 35 MW unit and the drilling of two exploratory wells as part of an extension to an additional 35 MW. The four deep wells drilled under this project so far and their measured depth from cellar top include: LA 11 D1 (2900m), LA 12 D2 (2690m), LA12 D3 (2740) and LA 13 D2 (2451m). The wells have been drilled directionally from 29 May 2021 to March 21, 2022 using two deep drilling rigs. The penetrated lithological units in these wells include; pyroclastic deposits (pumice, glass shards, variable silicic tuffs, lithic tuffs and rare vitreous tuff), rhyolites, volcanogenic lacustrine sediments, welded ignimbrites, basalts, scoria, intermediate rock units, fault breccia, crystal-rich ignimbrites and pre/syn rift basalt formation. Hydrothermal alteration facies observed in the wells showed evidence of prograde alteration sequence with five distinctive alteration zones with depth; shallow unaltered zone, smectites-zeolite zone, chlorite-Illite zone, epidote-Chlorite-Illite zone and actinolite-epidote-Chlorite-Illite zone.